In satellite systems consisting of several satellites, it is often desirable or necessary to know the distance between the satellites in order to aid the regulation of their positions and to prevent collisions. Although measuring from the ground, which has been practiced up to now, permits a comparatively exact distance measurement when two satellites are in close vicinity to each other, it is only possible with great elaboration.
For the distance determination necessary because of this between the satellites within a grouping placed into a tight sequence, its is possible to use known methods.
To be mentioned are
the measurement of the traveling time of a short pulse on the path to be measured between two satellites, PA1 the correlation of a known, but random-appearing sequence of data bits (pseudo-random sequence). PA1 no separate distance measuring device is required, PA1 a high degree of accuracy of the distance measurement is possible, PA1 the additional outlay for technical circuitry is low, and PA1 the additional output requirement is low.
Both methods require a transmission bandwidth which is inversely proportional to the required measurement resolution. Examples of solutions for both principles can be found in the current literature regarding radar technology. In every case an apparatus, which is separately intended for this purpose, requires a defined volume and has considerable weight for space travel applications and considerable energy requirements, which is considered to be a serious disadvantage.
For example, there exist optical apparatus and methods for the exact determination of signal traveling times or distances, wherein it is possible by means of a correlation between transmitted and received pseudo-random sequences to fall below the distance resolution given by the transmission bandwidth (H. Geiger et al., "New OTDR Technique for Monitoring the Range of Reflective Markers", 10th Optical Fiber Sensors Conference, p. 150 to 153). Applied to optical connections between satellites, there exists the additional property of the high angular resolution of an optical system.